Zinc oxide resistor and its manufacturing method

ABSTRACT

Disclosed are a zinc oxide resistor structure, and methods of forming a glass layer and a resistor, which are required for producing the resistor structure. The zinc oxide resistor comprises zinc oxide grains and an oxide glass layer which contains bismuth and boron and intervenes between the zinc oxide grains. The oxide glass layer residing between the zinc oxide grains changes the electric properties between the grains to achieve a higher resistance and a non-ohmic characteristic of a voltage-dependent resistance value in the resistor. This non-ohmic characteristic can be applied, particularly, to a non-ohmic device to be compatible with a low-voltage operation. Differently from conventional resistors, the oxide glass layer intervening between the zinc oxide grains can achieve an enhanced mechanical strength of a junction in the device.

TECHNICAL FIELD

The present invention relates to a zinc oxide resistor, and moreparticularly a varistor device structure for protecting anelectric/electronic circuit from surge voltages, and a production methodthereof.

BACKGROUND ART

1. Typical Zinc Oxide Varistor

Generally, a zinc oxide varistor is provided as a polycrystallinezinc-oxide ceramics. Specifically, the zinc oxide varistor has beenproduced by mixing zinc oxide powder, transition metal oxide powder andbismuth oxide powder, and burning the mixture at a high temperature, toform a polycrystalline body with a structure in which a bismuth oxide orthe like is segregated in the boundaries between zinc oxide grains eachcontaining a transition metal oxide dissolved therein in the form of asolid solution (see, for example, the following Non-Patent Publication1).

An appropriate additive makes it possible for the zinc oxide ceramics toexhibit a nonlinear current-voltage characteristic in which each grainboundary in the zinc oxide ceramics has an operating voltage of about 3V (see, for example, the following Non-Patent Publication 2). That is,the operating voltage as one of varistor characteristics is generallydetermined by the number of grain boundaries. Specifically, as shown inFIG. 7, in a varistor device comprising two electrodes 2A, 2B providedat opposite end surfaces thereof and zinc oxide (ZnO) grains 1 residingtherebetween, an overall operating voltage of the varistor device isdetermined by the product of the number of boundaries between the zincoxide grains 1 and the operating voltage having the nonlinearcurrent-voltage characteristic in each of the grain boundaries. Thus,the size of the zinc oxide grains 1 in the zinc oxide ceramicsinherently contributes to the varistor characteristics.

The grain size of a ceramics depends on an additive and a burningtemperature, and generally has a statistical distribution. This causesdifficulties in setting the number of grains or each size of grains in aceramics at a predetermined value. Therefore, the production of alow-voltage type varistor essentially requires a particular technique inaddition to a simple burning technique for forming a ceramics. Forexample, a varistor device having an operating voltage of 30 KV isrequired to ensure 10000 grain boundaries each having an operatingvoltage of 3 V. In contrast, a varistor device having an operatingvoltage of 6 V means a ceramics which includes 2 grain boundaries eachhaving an operating voltage of 3 V, or only 3 grains. That is, sometechnique for forming a ceramics having a small number of grainboundaries is required to produce a varistor operable at a low voltage.

2. Multilayer Varistor

As one technique for producing a varistor having a small number of grainboundaries, or a low operating voltage, there has been knownmultilayering (see, for example, the following Patent Publication 1).This technique comprises preparing a sheet-shaped compact to be burnedas a ceramics, and forming alternate layers of a metal electrode layerand a zinc-oxide ceramics layer, as shown in FIG. 8(A). The interveningmetal layer makes it possible to hinder the contact between the zincoxide grains or reduce the number of grain boundaries so as to achieve avaristor having a low operating voltage.

However, as seen in the enlarged view of FIG. 8(A), while each of agrain boundary 1 and a grain boundary 2 of the ZnO grains 1 extending ina direction orthogonal to a current path contributes to varistorcharacteristics, a grain boundary 3 extending parallel to a current pathwould be unnecessary in view of enhancement of varistor characteristics.Moreover, a current flowing along a current path P1 comes across threegrain boundaries, and a current flowing along a current path P2 comesacross four grain boundaries. This means that an operating voltage ofthe varistor differs between the current paths P1, P2. Thus, a varistoroperation at lower voltage can be achieved only if the number of grainboundaries is more strictly controlled.

3. Single-Grain-Boundary Varistor

It is known that each grain boundary of a zinc oxide varistor containingan appropriate additive has an operating voltage of 3 V. Thus, alow-voltage type varistor having any operating voltage of an integralmultiple of 3 V can be produced by preparing a plurality of varistorseach with a single grain boundary, and connecting them in series.

There has been known a single-grain-boundary varistor experimentallyproduced by forming a bismuth-containing oxide crystal phase interveningbetween zinc-oxide single crystals (see, for example, the followingNon-Patent Publication 3). While this technique can achieve acurrent-voltage characteristic with high nonlinearity, it still involvesa problem about strength of a junction between the opposed zinc-oxidesingle crystals. The single-grain-boundary varistor produced through aprocess utilizing a crystalline grain-boundary layer as disclosed in theNon-Patent Publication 3 leaves a problem about mechanical strength of ajunction therein, as in the after-mentioned Comparative Examples.

There has also been known a single-grain-boundary varistor deviceobtained by fundamentally joining zinc-oxide single crystals togetherwithout forming an intervening crystal phase between grain boundaries(see, for example, the following Non-Patent Publications 4 and 5). Whilea certain level of mechanical strength is achieved in the varistordevice disclosed in the Non-Patent Publication 4, the varistor devicewithout any intervening grain-boundary layer has a poor performance,wherein an α-value as a performance index of varistor characteristics isless than 10. Similarly, adequate varistor characteristics are notachieved in the varistor device disclosed in the Non-Patent Publication5, due to no intervening grain-boundary layer. However, the Non-PatentPublications 4 includes a valuable suggestion that, while a nonlinearcurrent-voltage characteristic is achieved by joining zinc-oxide singlecrystals each containing manganese and cobalt dissolved therein in theform of a solid solution, no nonlinear current-voltage characteristic isachieved if single crystals without addition of manganese and cobalt arejoined together.

[Patent Publication 1] Japanese Patent Laid-Open Publication No.10-270214

[Non-Patent Publication 1] M. Matsuoka, Jpn. J. Appl. Phys. 10, 736-746(1071)

[Non-Patent Publication 2] “Evaluation of Single Grain Boundaries inZnO: Rare-Earth Varistor by Micro-Electrodes” S. Tanaka, K. Takahashi;“Key Engineering Materials Series, Vol. 157-158, CSJ Series Vol. 1,(Electroceramics in Japan I)” p 241 (1998), (Trans Tech Publication,Switzerland)

[Non-Patent Publication 3] “MODEL EXPERIMENTS DESCRIBING THEMICRO-CONTACT OF ZnO VARISTORS”, SCHWINGU, HOFFMANN B, JOURNAL OFAPPLIED PHYSICS, 57 (12): 5372-5379 (1985)

[Non-Patent Publication 4] “Synthesis of ZnO bicrystals doped with Co orMn and their electrical properties” Ohashi N, Terada Y, Ohgaki T, TanakaS, Turumi T, Fukunaga O, Haneda H, Tanaka J, JAPANESE JOURNAL OF APPLIEDPHYSICS PART 1-REGULAR PAPER SHORT NOTES & REVIEW PAPERS, 38 (9A):5028-5032, SEPTEMBER (1999)

[Non-Patent Publication 5] “Current-voltage characteristic across [0001]twist boundaries in zinc oxide bicrystals” Sato Y, Oba F, Yamamoto T,Ikuhashi Y, Sakuma T, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, 85 (8):2142-2144, AUGUST (2002)

DISCLOSURE OF INVENTION

In view of the above circumstances, it is an object of the presentinvention to provide a single-grain-boundary varistor device or a zincoxide resistor exhibiting varistor characteristics based on anartificial grain boundary obtained by joining zinc-oxide singlecrystals.

It is an another object of the present invention to provide a zinc oxideresistor structure capable of achieving an α-value, or a performanceindex of a zinc oxide varistor, of about 20 or more, which is equivalentto that of a conventional typical polycrystalline varistor device, andjoining zinc-oxide single crystals with a high junction strength in anobtained artificial grain boundary, so as to eliminate the risk ofpeeling of the zinc-oxide single crystals during use.

It is yet another object of the present invention to provide a method ofproducing such a zinc oxide resistor.

The present invention provides a zinc oxide varistor having varistorcharacteristics achieved by a structure which comprises a pair ofopposed zinc-oxide single crystals each containing cobalt and manganesedissolved therein in the form of a solid solution, and a glass layerforming an oxide grain boundary layer which includes a primary componentconsisting of bismuth and boron and intervenes between the zinc-oxidesingle crystals.

The present invention provides a structure and production method forachieving zinc oxide varistor characteristics conventionally achieved ina zinc oxide sintered body, by a multilayer comprising a pair of opposedsingle crystals and a glass layer forming an oxide grain boundary layer.Thus, differently from a conventional varistor consisting ofpolycrystalline body, the technique of joining the opposed singlecrystals makes it possible to provide enhanced controllability of aresistor so as to obtain a varistor having an intended function.

In order to achieve the above objects, the following techniques areemployed.

(A) Use of Zinc-Oxide Single Crystals each containing Cobalt andManganese

A pair of zinc-oxide single crystals containing cobalt and manganesedissolved therein in the form of a solid solution are joined together toform a joined unit so as to provide a nonlinear current-voltagecharacteristic in a resistor having the joined zinc-oxide singlecrystals.

(B) Presence of Grain Boundary Layer containing Bismuth Oxide

Instead of simply joining the zinc-oxide single crystals, an interveninglayer containing bismuth oxide is formed in the junction interfacebetween the zinc-oxide single crystals to enhance the nonlinearcurrent-voltage characteristic of the zinc oxide varistor.

(C) Non-Crystallization of Grain Boundary Layer containing Bismuth Oxide

If the bismuth oxide layer intervening between the grain boundaries inthe joined unit of the zinc-oxide single crystals is crystallized, themechanical strength of the joined unit is likely to be spoiled, asdescribed above. Thus, the grain boundary layer of the joined unit isformed of a bismuth-and-boron-containing oxide glass phase. In a processof forming this glass phase, the boron oxide added to thebismuth-containing layer residing in the junction interface canaccelerate vitrification of the grain boundary layer by taking advantageof its feature of a low melting point.

The zinc-oxide varistor or resistor has a current-voltage characteristicwith significantly high nonlinearity, and a resistance value to bereduced in response to a high-voltage noise. Based on these features,the zinc-oxide resistor is used for protecting an electric/electroniccircuit from an abnormal high voltage. The junction interface betweenthe zinc-oxide single crystal/grain boundary layer/zinc-oxide singlecrystal distinctively provides significantly high nonlinearity at anoperating voltage of about 3 V. Thus, differently from the conventionalzinc-oxide varistor device consisting of sintered body orpolycrystalline body, in the zinc-oxide resistor of the presentinvention, the number of interfaces defined by the structure of(zinc-oxide single crystal/grain boundary layer/zinc-oxide singlecrystal) can be set at a desired value. Thus, the operating voltage fornoise removal can be readily adjusted.

For example, if it is necessary to protect an electric/electroniccircuit which is operated at 15 V but it is not warranted against anabnormal voltage of 20 V, six of the (zinc oxide layer/grain boundarylayer/zinc oxide layer) each having an operating voltage of 2.8 V can beelectrically connected in series to obtain a protection circuit capableof absorbing an abnormal voltage of 2.8×6=16.8 V or more.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a zinc oxide resistor of the presentinvention, which comprises a pair of opposed zinc-oxide single crystalsand an oxide glass interface layer containing bismuth and boron andintervening in the interface between the zinc-oxide single crystals.

FIG. 2 is a schematic diagram of a zinc-oxide resistor device with acontrolled operating voltage, produced by superimposing in a parallelarrangement the plurality of zinc oxide resistors of the presentinvention, which comprises a pair of opposed zinc-oxide single crystalsand an oxide glass interface layer containing bismuth and boron andintervening between the zinc-oxide single crystals.

FIG. 3 is a schematic diagram of a zinc-oxide resistor device with acontrolled operating voltage, produced by connecting in series theplurality of zinc oxide resistors of the present invention, whichcomprises a pair of opposed zinc-oxide single crystals and an oxideglass interface layer containing bismuth and boron and intervening inthe interface between the zinc-oxide single crystals.

FIG. 4 is an exemplary process diagram for producing the zinc oxideresistor of the present invention, which comprises a pair of opposedzinc-oxide single crystals and an oxide glass interface layer containingbismuth and boron and intervening in the interface between thezinc-oxide single crystals.

FIG. 5 is a graph showing a current-voltage characteristic at roomtemperature of a zinc oxide resistor obtained in Inventive Example 1.

FIG. 6 is a graph showing a current-voltage characteristic at roomtemperature of a zinc oxide resistor obtained in Inventive Example 2.

FIG. 7 is a schematic diagram of a conventional zinc oxide varistor.

FIG. 8 is a schematic diagram of a conventional multilayer zinc oxidevaristor.

BEST MODE FOR CARRYING OUT THE INVENTION

(1) A zinc oxide resistor of the present invention provides comprises asa basic unit a structure of (a zinc-oxide single crystal/a bismuth-boronbased oxide interface layer/a zinc-oxide single crystal) formed of apair of opposed zinc-oxide single crystals each containing cobalt andmanganese dissolved therein in the form of a solid solution, and anoxide which contains a primary component consisting of bismuth and boronand intervenes between the zinc-oxide single crystals. The zinc oxideresistor has non-ohmic properties or exhibits zinc-oxide varistorcharacteristics, based on the intervening oxide interface layer, and thebismuth-boron based oxide interface layer is formed as abismuth-and-boron-containing oxide glass phase by the action of theboron contained therein.

FIG. 1 shows a nonlinear resistor which has the so-called “varistorcharacteristics”, and a structure with only one grain boundary formed byjoining a pair of opposed single crystals. Specifically, the grainboundary is formed by disposing a pair of zinc-oxide single crystals 1A,1B each containing cobalt and manganese dissolved therein in the form ofa solid solution, in opposed relation to one another between a pair ofelectrodes 2A, 2B. The cobalt and manganese are elements which areconsidered to be essential to the expression of nonlinearity, asdisclosed in the aforementioned Non-Patent Publication 4.

As disclosed in the Non-Patent Publication 3, the nonlinearity can beenhanced by segregating a grain boundary layer containing bismuth, inthe grain boundary. Thus, an oxide layer 3 containing bismuth isprovided as an interface layer disposed between the zinc-oxide singlecrystals 1A, 1B. Additionally, in order to ensure a sufficientmechanical strength in the junction between the opposed zinc-oxidesingle crystals 1A, 1B or prevent the peeling of the opposed zinc-oxidesingle crystals 1A, 1B, the oxide layer 3 is formed as a glass phasecontaining bismuth oxide and boron oxide to provide enhanced junctionstrength.

When a zinc-oxide based nonlinear resistor is produced by the additionof bismuth, cobalt and manganese, an auxiliary additive, such asantimony, may be added to provide enhanced characteristics, as isgenerally known. While cobalt, manganese, bismuth and boron are used asadditives in the zinc-oxide resistor structure of the present invention,it is to be understood that any other suitable auxiliary additive may beadditionally used. The essence of the present invention lies in formingthe bismuth-and-boron-containing oxide glass phase in the junctionbetween the opposed zinc-oxide single crystals. Thus, it is notessential to the present invention whether an auxiliary additive otherthan cobalt and manganese is added to the opposed zinc-oxide singlecrystals.

In the above zinc oxide resistor of the present invention comprising asa basic unit a structure of (a zinc-oxide single crystal/a bismuth-boronbased oxide interface layer/a zinc-oxide single crystal) formed of apair of opposed zinc-oxide single crystals each containing cobalt andmanganese dissolved therein in the form of a solid solution, and anoxide which contains a primary component consisting of bismuth and boronand intervenes between the zinc-oxide single crystals, wherein the zincoxide resistor has non-ohmic properties or exhibits zinc-oxide varistorcharacteristics, based on the intervening oxide interface layer, and thebismuth-boron based oxide interface layer is formed as abismuth-and-boron-containing oxide glass phase by the action of theboron contained therein, (2) each of the opposed zinc-oxide singlecrystals may contain the cobalt dissolved therein in the form of a solidsolution, in an amount of 0.5 mol % or more with respect to zinctherein.

As described above, cobalt is one essential element to the expression ofnonlinearity. When it is necessary to achieve high nonlinearly in thezinc oxide resistor of the present invention having the interface layerformed as a glass phase containing a primary component consisting ofbismuth and boron, each of the opposed zinc-oxide single crystalspreferably contains the cobalt dissolved therein in the form of a solidsolution, in an amount of 0.5 mol % or more. In an actual productionprocess for the resistor, it is desirable that an optimal cobaltconcentration for achieving intended resistor characteristics isexperimentally determined while taking account of the type andconcentration of each impurity originally contained in zinc-oxide singlecrystals to be used. In this case, the solid solubility of cobalt intozinc oxide has a given limit, and the upper limit is determined by thesolid solubility of the cobalt in the zinc oxide.

In the above zinc oxide resistor of the present invention comprising asa basic unit a structure of (a zinc-oxide single crystal/a bismuth-boronbased oxide interface layer/a zinc-oxide single crystal) formed of apair of opposed zinc-oxide single crystals each containing cobalt andmanganese dissolved therein in the form of a solid solution, and anoxide which contains a primary component consisting of bismuth and boronand intervenes between the zinc-oxide single crystals, wherein the zincoxide resistor has non-ohmic properties or exhibits zinc-oxide varistorcharacteristics, based on the intervening oxide interface layer, and thebismuth-boron based oxide interface layer is formed as abismuth-and-boron-containing oxide glass phase by the action of theboron contained therein, (3) each of the opposed zinc-oxide singlecrystals may contain the manganese dissolved therein in the form of asolid solution, in an amount of 0.05 mol % or more with respect to zinctherein.

The addition of manganese can bring about an effect of reducing a leakcurrent due to the grain boundary to provide enhanced nonlinearity.

In the above zinc oxide resistor of the present invention comprising asa basic unit a structure of (a zinc-oxide single crystal/a bismuth-boronbased oxide interface layer/a zinc-oxide single crystal) formed of apair of opposed zinc-oxide single crystals each containing cobalt andmanganese dissolved therein in the form of a solid solution, and anoxide which contains a primary component consisting of bismuth and boronand intervenes between the zinc-oxide single crystals, wherein the zincoxide resistor has non-ohmic properties or exhibits zinc-oxide varistorcharacteristics, based on the intervening oxide interface layer, and thebismuth-boron based oxide interface layer is formed as abismuth-and-boron-containing oxide glass phase by the action of theboron contained therein, (4) each of the opposed zinc-oxide singlecrystals may have a length of 5 mm, a width of 5 mm, and a thickness of0.5 mm, and the oxide containing a primary component consisting ofbismuth and boron, to be used for forming a junction between the opposedzinc-oxide single crystals, may be a glass prepared in such a manner asto contain, in oxide wt % equivalent, 37.0 to 22.7 wt % of B₂O₃, 3.8 to1.9 wt % of Co₂O₃ and 5.7 to 1.6 wt % of MnO₂, with the remainder beingbismuth oxide.

The above composition of the interface layer is effective in producingthe zinc oxide resistor using zinc-oxide single crystals prepared bycutting a commercially-available zinc-oxide single crystal having atypical thickness of 0.5 mm, into a size of 5×5 mm, to achieve excellentjunction and nonlinearity. The present invention is not limited to theabove composition, but any other suitable composition may beappropriately selected depending on the thickness of each zinc-oxidesingle crystal to be used, the type and quantity of element originallydissolved in the zinc-oxide single crystals to be joined, in the form ofa solid solution, resistance characteristics required for the joinedunit, and other factors. In the same manner, a temperature and/ortime-period in the process of forming the junction may also beappropriately selected.

The above zinc oxide resistor of the present invention comprising as abasic unit a structure of (a zinc-oxide single crystal/a bismuth-boronbased oxide interface layer/a zinc-oxide single crystal) formed of apair of opposed zinc-oxide single crystals each containing cobalt andmanganese dissolved therein in the form of a solid solution, and anoxide which contains a primary component consisting of bismuth and boronand intervenes between the zinc-oxide single crystals, wherein the zincoxide resistor has non-ohmic properties or exhibits zinc-oxide varistorcharacteristics, based on the intervening oxide interface layer, and thebismuth-boron based oxide interface layer is formed as abismuth-and-boron-containing oxide glass phase by the action of theboron contained therein, (5) may exhibit an α-value of 20 or more, as aperformance index of a zinc oxide varistor.

Specifically, the zinc oxide resistor has the opposed zinc-oxide singlecrystals containing the cobalt and manganese dissolved therein in asolid solution, and the grain boundary layer having thebismuth-and-boron-containing oxide glass phase and intervening in thejoined unit. Particularly, based on this structure, the zinc oxideresistor exhibits an α-value of 20 or more, as a performance index of azinc oxide varistor.

In the above zinc oxide resistor of the present invention comprising asa basic unit a structure of (a zinc-oxide single crystal/a bismuth-boronbased oxide interface layer/a zinc-oxide single crystal) formed of apair of opposed zinc-oxide single crystals each containing cobalt andmanganese dissolved therein in the form of a solid solution, and anoxide which contains a primary component consisting of bismuth and boronand intervenes between the zinc-oxide single crystals, wherein the zincoxide resistor has non-ohmic properties or exhibits zinc-oxide varistorcharacteristics, based on the intervening oxide interface layer, and thebismuth-boron based oxide interface layer is formed as abismuth-and-boron-containing oxide glass phase by the action of theboron contained therein, (6) the (zinc-oxide singlecrystal/bismuth-boron based oxide interface layer/zinc-oxide singlecrystal) structure serving as the basic unit may have an operatingvoltage of 2.9±0.3 V, as a performance index of a zinc oxide varistor.

In the above zinc oxide resistor of the present invention comprising asa basic unit a structure of (a zinc-oxide single crystal/a bismuth-boronbased oxide interface layer/a zinc-oxide single crystal) formed of apair of opposed zinc-oxide single crystals each containing cobalt andmanganese dissolved therein in the form of a solid solution, and anoxide which contains a primary component consisting of bismuth and boronand intervenes between the zinc-oxide single crystals, wherein the zincoxide resistor has non-ohmic properties or exhibits zinc-oxide varistorcharacteristics, based on the intervening oxide interface layer, and thebismuth-boron based oxide interface layer is formed as abismuth-and-boron-containing oxide glass phase by the action of theboron contained therein, (7) the (zinc-oxide singlecrystal/bismuth-boron based oxide interface layer/zinc-oxide singlecrystal) structure may be provided in a number of n. In this case, thestructures of the number n may be repeatedly superimposed in a layeredmanner, and provided with a zinc-oxide single crystal superimposedthereon to form a (n+1) layered structure including the number (n+1) ofzinc-oxide single crystals and the number n of bismuth-boron based oxideinterface layers. The zinc-oxide resistor has an operating voltage of(2.9±0.3) n V, as a performance index of a zinc oxide varistor.

Specifically, the operating voltage of 2.9±0.3 V is achieved by a singlejunction, and the structures of the number n can be superimposed in alayered manner as shown in FIG. 2 to provide a multilayer zinc oxideresistor having any desired operating voltage.

In the above zinc oxide resistor of the present invention comprising asa basic unit a structure of (a zinc-oxide single crystal/a bismuth-boronbased oxide interface layer/a zinc-oxide single crystal) formed of apair of opposed zinc-oxide single crystals each containing cobalt andmanganese dissolved therein in the form of a solid solution, and anoxide which contains a primary component consisting of bismuth and boronand intervenes between the zinc-oxide single crystals, wherein the zincoxide resistor has non-ohmic properties or exhibits zinc-oxide varistorcharacteristics, based on the intervening oxide interface layer, and thebismuth-boron based oxide interface layer is formed as abismuth-and-boron-containing oxide glass phase by the action of theboron contained therein, (8) the (zinc-oxide singlecrystal/bismuth-boron based oxide interface layer/zinc-oxide singlecrystal) structure may be adjusted to have an operating voltage of x V,as a performance index of a zinc oxide varistor, and provided in anumber of n. In this case, the structures of number n may beelectrically connected in series. The zinc-oxide resistor has anoperation voltage of n×x V, as a performance index of a zinc oxidevaristor.

Specifically, the operating voltage of 2.9±0.3 V is achieved by a singlejunction, and the structures of the number n can be electricallyconnected to each other using lead wires as shown in FIG. 3 to provide aseries-connected zinc oxide resistor having any desired operatingvoltage.

(9) A method of producing either one of the above zinc oxide resistors(1) to (7) of the present invention, comprises: disposing an oxidecontaining bismuth and boron, between a pair of opposed zinc-oxidesingle crystals to form a sandwich structure of (a zinc-oxide singlecrystal/a composition to be formed as a glass phase/a zinc-oxide singlecrystal); heating and holding the sandwich structure at a hightemperature allowing the oxide containing bismuth and boron, to bemolten; and rapidly cooling the heated sandwich structure to join thepair of zinc-oxide single crystals with a glass-phase oxide interfacelayer intervening therebetween.

In this method, a process of disposing the bismuth-and-boron-containingoxide between the opposed zinc-oxide single crystals, to form thesandwich structure of (a zinc-oxide single crystal/a composition to beformed as a glass phase/a zinc-oxide single crystal), includes aplurality of options. FIG. 4 shows this process. In one of the options,a suitable material for forming a desired glass phase is molten andvitrified in advance using a crucible, such as a platinum crucible, andthen crushed to obtain a glass powder. In another option, a compositionto be formed as a desired glass phase is placed on one zinc-oxide singlecrystal to be joined, and the zinc-oxide single crystal is used as aplate for vitrification without using a crucible. In the former case,the obtained glass phase is disposed between a pair of zinc-oxide singlecrystals to be joined, and then they are subjected to a heat treatmentto form a junction between the zinc-oxide single crystals. In the lattercase, another zinc-oxide single crystal is disposed in opposed relationto the zinc-oxide single crystal having the vitrified compositionattached thereon, and they are subjected to a heat treatment to form ajunction between the zinc-oxide single crystals.

In this process, a time-period of the heat treatment for heating thesandwich structure at a high temperature to melt the glass and form thejunction is not limited to a specific value. While the heat-treatmenttime is required to set at a sufficient value for allowing the glass tobe homogenized, an excessive heat-treatment time induces a reactionbetween the glass components and the zinc-oxide single crystals to causeelution of zinc oxide into the glass components. Thus, it is practicallydesirable to prepare a glass powder in advance as in the former case,and, after heating the glass powder at a high temperature allowing theglass to be molten, for a melting time of about 3 to 12 hours, rapidlycool the glass.

In either case, it is desirable to optimize the composition of the glassphase in consideration of wettability to zinc oxide so as to obtain theaforementioned required structure of the zinc oxide resistor. Forexample, the glass composition is preferably determined by synthesizinga glass phase containing bismuth and boron, melting and rapidly coolingthe glass on the zinc-oxide single crystal to form a joined unit of zincoxide and glass droplets, calculating a contact angle between the glassand the zinc-oxide single crystal based on this joined unit, selecting aglass allowing the contact angle to be 5 degrees or less.

In the above production method, it is not specified whether cobalt andmanganese are added to the glass. If each of the zinc-oxide singlecrystals to be joined has a thin-plate shape, cobalt and manganese maybe added in advance to glass components to be formed as the grainboundary of the joined unit, and diffused in the zinc-oxide singlecrystals in conjunction with the process of subjecting the sandwichstructure to a heat treatment at a high temperature to melt the glassand form the junction.

If each of the zinc-oxide single crystals to be joined has a thick-plateshape, the heat treatment time for forming the junction is likely tocause insufficient diffusion of the cobalt and manganese from the moltenglass to the zinc oxide. Thus, irrelevant to each thickness of thezinc-oxide single crystals, the above production method is preferablyimplemented in such a manner that a pair of zinc-oxide single crystalseach containing cobalt and manganese diffused therein in advance is usedtogether with the glass components to formed the sandwich structure, andthen the sandwich structure is subjected to a heat treatment at a hightemperature to form the junction.

The above production method is only one typical example. Thus, any othersuitable method may be used to produce the above zinc oxide resistorcomprising as a basic unit a structure of (a zinc-oxide single crystal/abismuth-boron based oxide interface layer/a zinc-oxide single crystal)formed of a pair of opposed zinc-oxide single crystals each containingcobalt and manganese dissolved therein in the form of a solid solution,and an oxide which contains a primary component consisting of bismuthand boron and intervenes between the zinc-oxide single crystals, whereinthe zinc oxide resistor has non-ohmic properties or exhibits zinc-oxidevaristor characteristics, based on the intervening oxide interfacelayer, and the bismuth-boron based oxide interface layer is formed as abismuth-and-boron-containing oxide glass phase by the action of theboron contained therein. That is, the above zinc oxide resistor of thepresent invention comprising as a basic unit a structure of (azinc-oxide single crystal/a bismuth-boron based oxide interface layer/azinc-oxide single crystal) formed of a pair of opposed zinc-oxide singlecrystals each containing cobalt and manganese dissolved therein in theform of a solid solution, and an oxide which contains a primarycomponent consisting of bismuth and boron and intervenes between thezinc-oxide single crystals, wherein the zinc oxide resistor hasnon-ohmic properties or exhibits zinc-oxide varistor characteristics,based on the intervening oxide interface layer, and the bismuth-boronbased oxide interface layer is formed as a bismuth-and-boron-containingoxide glass phase by the action of the boron contained therein, is notlimited by a production method thereof.

(10) A method of producing either one of the above zinc oxide resistors(1) to (7) of the present invention, comprises: bringing each of twozinc-oxide single crystals into contact with a chunk of oxide cobalt,and heating the zinc-oxide single crystals and the chunk of oxide cobaltat a high temperature capable of inducing a diffusion reaction todiffuse cobalt from the chunk of oxide cobalt into the zinc-oxide singlecrystals so as to prepare a pair of zinc-oxide single crystals in such amanner as to have a cobalt concentration of 0.5 mol % or more; disposingan oxide containing bismuth and boron, between the pair of zinc-oxidesingle crystals disposed opposed to one another, to form a sandwichstructure of (a zinc-oxide single crystal/a composition to be formed asa glass phase/a zinc-oxide single crystal); heating and holding thesandwich structure at a high temperature allowing thebismuth-and-boron-containing oxide to be molten; and rapidly cooling theheated sandwich structure to join the pair of zinc-oxide single crystalswith a glass-phase oxide interface layer intervening therebetween.

Specifically, in order to obtain enhanced characteristics in the zincoxide resistor through the production method of the present invention,it is required to adjust the concentration of cobalt in the zinc-oxidesingle crystals to be joined, at a desired value. Thus, if acommercially-available zinc-oxide single crystal containing no cobalt,it is necessary to introduce an appropriate amount of cobalt into thesingle crystal. In this process of introducing cobalt, it is desirableto introduce a high concentration of cobalt without roughening thesurface of the zinc-oxide single crystal.

For this purpose, cobalt can be conveniently introduced into azinc-oxide single crystal by preparing a chunk containing a primarycomponent consisting of oxide cobalt, bringing the chunk of oxide cobaltinto contact with a target zinc-oxide single crystal, and heating andholding them at a high temperature. A measurement based on lightabsorption spectrum may be conveniently used for determining a quantityof cobalt to be introduced, in a nondestructive manner. In this case, ananalytical curve may be made in advance to calculate a quantity ofcobalt to be added, based on light absorption spectrum.

(11) In the production method (9) or (10) of the present invention, wheneach of the opposed zinc-oxide single crystals has a length of 5 mm, awidth of 5 mm, and a thickness of 0.5 mm, the oxide containing a primarycomponent consisting of bismuth and boron, to be used for forming ajunction between the opposed zinc-oxide single crystals, may be a glassprepared in such a manner as to contain, in oxide wt % equivalent, 37.0to 22.7 wt % of B₂O₃, 3.8 to 1.9 wt % of Co₂O₃ and 5.7 to 1.6 wt % ofMnO₂, with the remainder being bismuth oxide.

Specifically, each optimal quantity of cobalt, manganese, bismuth andboron to be contained in the glass phase differs depending on each sizeof the opposed zinc-oxide single crystals to be joined, and the type andquantity of additives originally containing therein. Thus, the abovecomposition is simply shown as one example capable of producing anonlinear resistor using the zinc-oxide single crystals each having alength of 5 mm, a width of 5 mm, and a thickness of 0.5 mm. Therefore,the above composition is not always an applicable value to all of zincoxide resistors having the interface layer formed as abismuth-and-boron-containing oxide glass phase according to the presentinvention.

(12) In either one of the production methods (9) to (11) of the presentinvention, the oxide containing a primary component consisting ofbismuth and boron, to be used for forming a junction between the opposedzinc-oxide single crystals, may be a glass. In this case, the method mayinclude flattening each surface of the opposed zinc-oxide singlecrystals through mirror polishing, and adjusting a quantity of the glassin such a manner that a molar ratio of the glass quantity in anequivalent bismuth quantity contained in the glass to a quantity of theopposed zinc-oxide single crystals, is set at 1.2 mol %.

This total quantity of the glass phase for forming the junction simplyshows a desirable value in one case where each surface of the opposedzinc-oxide single crystals is flatted through mirror polishing.Fundamentally, it is desirable that the total quantity of the glassphase for forming the junction is optimized in consideration of eachflatness or surface area of the opposed zinc-oxide single crystals to bejoined. Thus, in advance of an actual production, it is desirable todetermine an optimal amount of bismuth with reference to the aboverecommended value.

EXAMPLE Inventive Example 1

Each of two zinc-oxide single crystals was in contact with acobalt-oxide sintered body, in an oxygen flow at 1200° C. for 3 hours todiffuse cobalt into each zinc-oxide single crystal so as to prepare twocobalt-doped zinc-oxide single crystals. A quantity of the resultingsolid solution of cobalt was calculated as about 1 at % based on opticalspectrum. Then, 0.8772 g of boron oxide, 8.8068 g of bismuth oxide,0.1517 g of cobalt oxide and 0.16431 g of manganese oxide were measuredand mixed together. The obtained mixture was put in a platinum crucible,and molten at 900° C. in an oxygen flow. Then, the molten mixture wasflowed out of the crucible, and solidified to obtain abismuth-and-boron-containing oxide glass. After crushing the glass, theobtained glass powder was dredged on one of the prepared cobalt-dopedzinc-oxide single crystals (5×5×0.5 mm), and another zinc-oxide singlecrystal was superimposed on the single crystal with the glass powder toform a sandwich structure.

Without particular pressing, the sandwich structure was heated at 1000°C. in an oxygen flow for 12 hours, and then cooled to room temperatureover a period of about 5 hours to produce a zinc oxide resistor. Themanganese was dissolved in the zinc-oxide single crystals throughdiffusion. As shown in FIG. 5, the obtained zinc oxide resistorexhibited a current-voltage characteristic represented by α=20.

Inventive Example 2

Each of two zinc-oxide single crystals was in contact with acobalt-oxide sintered body, in an oxygen flow at 1200° C. for 12 hoursto diffuse cobalt into each zinc-oxide single crystal so as to preparetwo cobalt-doped zinc-oxide single crystals. Then, 0.8772 g of boronoxide, 8.8068 g of bismuth oxide, 0.1517 g of cobalt oxide and 0.16431 gof manganese oxide were measured and mixed together. The obtainedmixture was put in a platinum crucible, and molten at 900° C. in anoxygen flow. Then, the molten mixture was flowed out of the crucible,and solidified to obtain a bismuth-and-boron-containing oxide glass.After crushing the glass, the obtained glass powder was dredged on oneof the prepared cobalt-doped zinc-oxide single crystals (5×5×0.5 mm),and another zinc-oxide single crystal was superimposed on the singlecrystal with the glass powder to form a sandwich structure.

Without particular pressing, the sandwich structure was heated at 1000°C. in an oxygen flow for 4 hours, and then cooled to room temperatureover a period of about 5 hours to produce a zinc oxide resistor. Themanganese was dissolved in the zinc-oxide single crystals throughdiffusion. As shown in FIG. 6, the obtained zinc oxide resistorexhibited a current-voltage characteristic represented by α=26.

Comparative Example 1

9.5762 g of bismuth oxide, 0.2749 g of cobalt oxide and 0.1489 g ofmanganese oxide were measured and mixed together. The obtained mixturewas put in a platinum crucible, and molten at 900° C. in an oxygen flow.Then, the molten mixture was flowed out of the crucible, and solidifiedto obtain a bismuth-containing oxide glass without boron. Through x-raydiffraction measurement, it was proven that the obtained oxide is acrystal phase. After crushing the bismuth-containing oxide glass, theobtained glass powder was dredged on one of two cobalt-doped zinc-oxidesingle crystals (5×5×0.5 mm) prepared in advance, and another zinc-oxidesingle crystal was superimposed on the single crystal with the glasspowder to form a sandwich structure.

Without particular pressing, the sandwich structure was heated at 1000°C. in an oxygen flow for 1 hour, and then cooled to room temperatureover a period of about 5 hours to produce a zinc oxide resistor. While ameasurement about characteristics was attempted in the same manner asthat in Incentive Examples 1 and 2, the zinc-oxide single crystals waspeeled from one another during the measurement due to poor junctionstrength. The reason would be that the interface layer is formed aspolycrystal due to no addition of boron into the interface layer, andconsequently grain boundaries and/or cracks are formed in the interfacelayer to cause deterioration in mechanical strength.

Comparative Example 2

0.6018 g of boron oxide and 9.3982 g of bismuth oxide were measured andmixed together. The obtained mixture was put in a platinum crucible, andmolten at 900° C. in an oxygen flow. Then, the molten mixture was flowedout of the crucible, and solidified to obtain abismuth-and-boron-containing oxide glass. After crushing thebismuth-and-boron-containing oxide glass, the obtained glass powder wasdredged on one of two cobalt-doped zinc-oxide single crystals (5×5×0.5mm) prepared in advance without a cobalt diffusion treatment, andanother zinc-oxide single crystal was superimposed on the single crystalwith the glass powder to form a sandwich structure.

Without particular pressing, the sandwich structure was heated at 1000°C. in an oxygen flow for 4 hours, and then cooled to room temperatureover a period of about 5 hours to produce a zinc oxide resistor. While ameasurement about characteristics was attempted in the same manner asthat in Incentive Examples 1 and 2, a linear current-voltagecharacteristic was obtained without any observation of a nonlinearcurrent-voltage characteristic

INDUSTRIAL APPLICABILITY

Differently from a high-voltage varistor as typified by arrestors, thezinc oxide resistor of the present invention is applicable to alow-voltage varistor, and usable in removing low-voltage noises inelectronic components.

1. A zinc oxide resistor comprising as a basic unit a structure of (azinc-oxide single crystal/a bismuth-boron based oxide interface layer/azinc-oxide single crystal) formed of a pair of opposed zinc-oxide singlecrystals each containing cobalt and manganese dissolved therein in theform of a solid solution, and an oxide which contains a primarycomponent consisting of bismuth and boron and intervenes between saidzinc-oxide single crystals, wherein said zinc oxide resistor hasnon-ohmic properties or exhibits zinc-oxide varistor characteristics,based on said intervening oxide interface layer, and said bismuth-boronbased oxide interface layer is formed as a bismuth-and-boron-containingoxide glass phase by the action of said boron contained therein.
 2. Thezinc oxide resistor as defined in claim 1, wherein each of said opposedzinc-oxide single crystals contains said cobalt dissolved therein in theform of a solid solution, in an amount of 0.5 mol % or more with respectto zinc therein.
 3. The zinc oxide resistor as defined in claim 1,wherein each of said opposed zinc-oxide single crystals contains saidmanganese dissolved therein in the form of a solid solution, in anamount of 0.05 mol % or more with respect to zinc therein.
 4. The zincoxide resistor as defined in claim 1, wherein: each of the opposedzinc-oxide single crystals has a length of 5 mm, a width of 5 mm, and athickness of 0.5 mm; said oxide containing a primary componentconsisting of bismuth and boron, to be used for forming a junctionbetween said opposed zinc-oxide single crystals, is a glass prepared insuch a manner as to contain, in oxide wt % equivalent, 37.0 to 22.7 wt %of B₂O₃, 3.8 to 1.9 wt % of Co₂O₃ and 5.7 to 1.6 wt % of MnO₂, with theremainder being bismuth oxide.
 5. The zinc oxide resistor as defined inclaim 1, which exhibits an α-value of 20 or more, as a performance indexof a zinc oxide varistor.
 6. The zinc oxide resistor as defined in claim1, wherein said (zinc-oxide single crystal/bismuth-boron based oxideinterface layer/zinc-oxide single crystal) structure serving as saidbasic unit has an operating voltage of 2.9±0.3 V, as a performance indexof a zinc oxide varistor.
 7. The zinc oxide resistor as defined in claim1, wherein said (zinc-oxide single crystal/bismuth-boron based oxideinterface layer/zinc-oxide single crystal) structure is provided in anumber of n, wherein said structures of the number n are repeatedlysuperimposed in a layered manner, and provided with a zinc-oxide singlecrystal superimposed thereon to form a (n+1) layered structure includingthe number (n+1) of zinc-oxide single crystals and the number n ofbismuth-boron based oxide interface layers, wherein said zinc-oxideresistor has an operating voltage of (2.9±0.3) n V, as a performanceindex of a zinc oxide varistor.
 8. The zinc oxide resistor as defined inclaim 1, wherein said (zinc-oxide single crystal/bismuth-boron basedoxide interface layer/zinc-oxide single crystal) structure is adjustedto have an operating voltage of x V, as a performance index of a zincoxide varistor, and provided in a number of n, wherein said structuresof number n are electrically connected in series, wherein saidzinc-oxide resistor has an operation voltage of n×x V, as a performanceindex of a zinc oxide varistor.
 9. A method of producing the zinc oxideresistor as defined in claim 1, comprising: disposing an oxidecontaining bismuth and boron, between a pair of opposed zinc-oxidesingle crystals to form a sandwich structure of (a zinc-oxide singlecrystal/a composition to be formed as a glass phase/a zinc-oxide singlecrystal); heating and holding said sandwich structure at a hightemperature allowing said oxide containing bismuth and boron, to bemolten; and rapidly cooling said heated sandwich structure to join saidpair of zinc-oxide single crystals with a glass-phase oxide interfacelayer intervening therebetween.
 10. The method as defined in claim 9,includes: bringing each of two zinc-oxide single crystals into contactwith a chunk of oxide cobalt, and heating said zinc-oxide singlecrystals and said chunk of oxide cobalt at a high temperature capable ofinducing a diffusion reaction to diffuse cobalt from said chunk of oxidecobalt into said zinc-oxide single crystals so as to prepare each ofsaid opposed zinc-oxide single crystals in such a manner as to have acobalt concentration of 0.5 mol % or more.
 11. The method as defined inclaim 9, wherein: each of the opposed zinc-oxide single crystals has alength of 5 mm, a width of 5 mm, and a thickness of 0.5 mm; said oxidecontaining a primary component consisting of bismuth and boron, to beused for forming a junction between said opposed zinc-oxide singlecrystals, is a glass prepared in such a manner as to contain, in oxidewt % equivalent, 37.0 to 22.7 wt % of B₂O₃, 3.8 to 1.9 wt % of Co₂O₃ and5.7 to 1.6 wt % of MnO₂, with the remainder being bismuth oxide.
 12. Themethod as defined in claim 9, wherein said oxide containing a primarycomponent consisting of bismuth and boron, to be used for forming ajunction between said opposed zinc-oxide single crystals, is a glass,wherein said method includes: flattening each surface of said opposedzinc-oxide single crystals through mirror polishing; and adjusting aquantity of said glass in such a manner that a molar ratio of said glassquantity in an equivalent bismuth quantity contained in said glass to aquantity of said opposed zinc-oxide single crystals, is set at 1.2 mol%.